1. Field of the Invention
This invention relates to polyisocyanates containing acylurea groups, a process for their preparation and their use as starting components in the production of polyurethane plastics, in particular, as cross-linking agents for a binder for coating compositions that contain isocyanate-reactive groups.
2. Description of the Prior Art
The reaction of isocyanates with carboxylic acids is a reaction which previously has not been extensively investigated. The formation of isocyanate polyaddition products containing acylurea groups is described, for example, in P. Babusiaux, R. Longeray, J. Dreux, Liebigs Ann. Chem., 1976, (3), 487, A. H. M. Schotmann, W. J. Mijs, Recl. Trav. Chim. Pays Bas 1992, 111, 88, DE-A 2 436 740 and the literature cited in this work). The condensation reaction of isocyanates with carboxylic acids was also investigated by I. S. Blagbrough, N. E. Mackenzie, C. Ortiz, A. and I. Scott (Tetrahedron Lett. 1986, 27(11), 1251).
The preparation of acylureas by the reaction of amides with isocyanates has also been described, for example, J. T. Micich (J. Am. Oil Chem. Soc. 1982, 59(2), 92–4), which describes the synthesis of acylureas obtained by the reaction of carboxylic acid amides with isocyanates.
German Auslegeschrift DE-A 1 230 778 describes a process for preparing acylated urea polyisocyanates. The procedure starts with monocarboxylic acids, which are added in high dilution to a very large excess of the diisocyanate component. The disadvantages of this process are the high reaction temperatures (approximately 130–140° C.) and the long period of time, lasting approximately 5 hours, over which the monocarboxylic acid is added to the isocyanate. The processes cited are unsuitable for the preparation of polyisocyanates that are to be suitable for applications in the field of coatings because the products yellow due to the high reaction temperatures.
A process for producing plastics containing amide groups is described in EP-A 0 624 170. Aliphatic and aromatic polyisocyanates are reacted with polyfunctional carboxylic acids (at least dicarboxylic acids) and polyalcohols to form plastics. Tertiary amines, such as aminosubstituted pyridines and/or N-substituted imidazoles, are used as catalysts. Nothing is stated regarding the selectivities of this reaction or the secondary reactions. The products obtained are yellow foamed plastics. It is to be assumed from this that in this process the catalyst has an influence on the (yellow) color of the resulting product. Basic tertiary amines, amidines and imidazoles in particular are readily oxidizable and in the course of relatively long reaction times may lead to a yellowing of the reaction batch.
The previous investigations have shown that only the reaction of carboxylic acids with isocyanates, with the aim of producing foamed plastics, leads to colorless products. Apart from that, no conditions which deliver colorless or slightly colored light-fast polyisocyanates by carrying out the reaction at low temperatures and within technically achievable reaction times have been found.
Consequently, from none of the above-mentioned publications was a person skilled in the art able to infer any definite information as to how a catalyst for the formation of reaction products of isocyanates with carboxylic groups, accompanied by liberation of CO2, should be constituted in order to obtain colorless products.
An object of the present invention is to find a process which leads to colorless, unfoamed products, such as polyisocyanates, by modifying the reaction of isocyanates with carboxylic acids.
Surprisingly, it has now been found that by using a suitable catalyst and suitable reaction conditions, the reaction temperatures and/or the reaction time for the reaction of carboxylic acids with isocyanates can be significantly lowered. The process according to the invention is based on the reaction of carboxylic acids with isocyanates with the release of CO2, a catalyst being present in order to achieve a rapid reaction leading to colorless products. In this process, the reaction of the carboxylic acids with isocyanates produces first of all amides, which then form so-called acylureas by reaction with a further isocyanate group.